1. Field of the Invention
The present invention relates to a process for producing an oligomer. More particularly, it relates to a process for efficiently producing an oligomer suitable for use in a hydrocarbon-based traction drive fluid, which has good flowability at low temperatures and exhibits high performance in a wide temperature range from low to high temperatures.
2. Description of the Related Arts
Generally, traction drive fluids are used in traction drive units (friction driving equipment utilizing rolling contact), for example, continuously variable transmission for automobiles, continuously variable transmission for industrial purposes, and hydraulic machines. As the demands for drive fluids have increased, it is required that traction drive fluids should have a high traction coefficient, good stability to heat and oxidation and a high economical efficiency.
In recent years, attempts have been made to construct small-sized and lightweight traction drive units for the sake of, mainly, the automobile use. Therefore, traction drive fluids to be used in the traction drive units are required to have performance capable of withstanding the use under various severe conditions, particularly to exhibit high performance (e.g., a high traction coefficient, a low viscosity, and good stability to heat and oxidation) reliably over a wide temperature range from low to high temperatures (approximately -30.degree. to +140.degree. C.).
Various traction drive fluids have heretofore been proposed in, for example, Japanese Patent Publication Nos. 46(1971)-338, 46(1971)-339, and 61(1986)-44918. However, any of these proposed traction drive fluids cannot satisfy the requirements described above and have various problems. For example, a compound exhibiting a high traction coefficient at high temperatures has the drawbacks in that it causes large churning loss to occur due to its high viscosity, and therefore the transmission efficiency is low, and in that starting characteristics at low temperatures are bad. A compound having a low viscosity and a high transmission efficiency has the drawbacks in that the traction coefficient at high temperatures is markedly low, in that the viscosity becomes markedly low at high temperatures and causes a trouble in lubrication of the traction drive units.
In view of the above circumstances, the research group of the inventors carried out study to eliminate the problems of the conventional techniques and found that a hydrogenated oligomer of an unsaturated bicyclo heptane derivative (which is proposed in Japanese Patent Application Laid-Open No. 3(1991)-95295) and a hydrogenated oligomer of an unsaturated bicyclo octane derivative (which is proposed in Japanese Patent Application Laid-Open No. 5(1993)-9134) had good performance as traction drive fluids for lubrication of traction drive units.
When the hydrogenated oligomer of an unsaturated bicyclo heptane derivative and the hydrogenated oligomer of an unsaturated bicyclo octane derivative described above are produced, it is necessary that, prior to hydrogenation, the unsaturated bicyclo heptane derivative and the unsaturated bicyclo octane derivative should be subjected to oligomerization. The unsaturated bicyclo heptane derivative and the unsaturated bicyclo octane derivative, which are each the raw materials for the oligomers, are obtained from dehydration reactions of respective corresponding alcohols.
During the dehydration reaction of the corresponding alcohol, cyclopentadiene derivatives or cyclohexadiene derivatives inevitably occur as by-products. Therefore, when the unsaturated bicyclo heptane derivative or the unsaturated bicyclo octane derivative, which is obtained from the dehydration reaction of the corresponding alcohol, is subjected to oligomerization, a raw material containing a cyclopentadiene derivative or a cyclohexadiene derivative as the by-products of the dehydration reaction is supplied to the oligomerization step.
However, it has been found that, if a cyclodiene derivative occurring as the by-product of the dehydration reaction is present in the raw material, the oligomerization of the unsaturated bicyclo heptane derivative or the unsaturated bicyclo octane derivative cannot proceed smoothly.
Accordingly, the inventors eagerly carried out study to eliminate the above-described problems.
As a result, it has been found that, in the cases where the formation of the cyclopentadiene derivatives or the cyclohexadiene derivatives as by-products is restricted to at most 1% by weight when the unsaturated bicyclo heptane derivative or the unsaturated bicyclo octane derivative is produced by the dehydration reaction of the corresponding alcohol, and in cases where the thus obtained compound is used as the raw material for the oligomerization, the oligomerization reaction can proceed smoothly. The present invention has been accomplished based on such findings.